The aim of this proposal is to uncover functional relationships between neuroanatomical change and vocal learning in passerine songbirds. In zebra finches, song behavior is androgen-dependent, and normally produced only by males. Vocal development involves a sensory phase during which birds memorize a song model, and a sensorimotor phase when birds use auditory feedback to match their own vocalizations to this memory. Each phase occurs during a restricted developmental period which, in male zebra finches, overlaps with dramatic changes in neuron number and connectivity among the neural regions controlling song. The studies proposed have two specific goals; 1) to determine how ontogenetic changes in the anatomy of song-related brain regions influence, or are influenced by vocal development, and 2) to assess or better define the role of specific neural regions in vocal learning. Much of the proposed research will employ females that have been masculinized by steroids shortly after hatching. These females exhibit ontogenetic changes in song regions that mimic those in males, and develop song when stimulated with androgens. Importantly, sensorimotor learning in masculinized females can be offset from these neural changes by manipulating the timing of androgen exposure. Hormonal manipulations and behavioral analyses will be used to determine if the critical period for sensory learning is terminated by androgenic stimulation of vocal practice, or by ontogenetic changes in song-related neural regions. Behavioral analyses will also be used to determine if overlap between ontogenetic changes in neural structure and sensorimotor learning facilitates song learning. Light microscopic analyses and anterograde amino acid autoradiography will be used to determine whether androgens and/or sensorimotor learning augment changes in neuron number or projections of song-control nuclei. In order to identify neural changes unique to song learning, androgenic effects on neural structure will be assessed in masculinized females deafened to prevent sensorimotor learning. Finally, electrolytic and neurochemical lesions will be performed to further our understanding of the functional role of several song nuclei. Striking similarities between vocal development in songbirds and language acquistion in humans argue that a better understanding of neural mechanisms of song learning may provide insight into neural pathologies underlying learning disorders in general, and more specifically, abnormalities in language acquistion.